The invention relates to friction clutches in general, especially for use in motor vehicles, and more particularly to improvements in friction clutches of the type wherein a pressure plate is movable axially by a diaphragm spring so as to bias a clutch plate against a flywheel or another rotary part.
It is known to tiltably mount the diaphragm spring of a friction clutch at the inner side of a housing or cover which can be made of metallic sheet material and is rotated by a flywheel to thereby rotate an axially movable pressure plate which can be biased by the diaphragm spring in a direction toward the flywheel whereby a clutch plate of the friction clutch is clamped between the pressure plate and the flywheel and can transmit torque to the input shaft of a variable-speed transmission in a motor vehicle. The flywheel is driven by the engine, either directly or through the medium of a second flywheel.
When the friction clutch is engaged or disengaged, vibrations and/or other stray movements which are or can be caused by the running engine are transmitted to the pressure plate. The mass of the pressure plate is rather pronounced, and the aforementioned vibrations cause the pressure plate to wobble relative to the diaphragm spring and/or to perform other axial (e.g., vibratory) movements in directions toward and away from the diaphragm spring. It has been found that stray movements of the pressure plate relative to the diaphragm spring are especially likely to develop in response to partial or complete disengagement of the friction clutch, i.e., in response to partial or complete interruption of transmission of torque between the engine and the transmission. This entails repeated lifting of the pressure plate off and repeated propulsion of the pressure plate against the diaphragm spring. The result is a hammering or rattling noise which is transmitted to other parts such as the clutch pedal and the chassis of the motor vehicle. The thus amplified noise causes discomfort to the occupant or occupants of the vehicle.
Attempts to reduce noise which is caused by the pressure plate during disengagement of a friction clutch include the provision of so-called lifting yokes which are interposed between the diaphragm spring and the pressure plate and perform the additional function of disengaging the pressure plate from the diaphragm spring. To this end, the yokes are mounted on the pressure plate in such a way that they cause the pressure plate to move axially and away from the diaphragm spring when the latter is tilted relative to the clutch housing during engagement or disengagement of the friction clutch. Reference may be had, for example, to FIG. 9 of German Pat. No. 12 33 670.
It has been found that the just discussed yokes cannot be put to use in all types of friction clutches because they fail to invariably disengage the pressure plate from the diaphragm spring so that the pressure plate can initiate the generation of undesirable and often unacceptable noise in response to partial or complete disengagement of the friction clutch.